Even at age 64, kicking the habit can add four years to a person's life, while quitting by age 34 can increase life expectancy by a decade, according to a study published online by the New England Journal of Medicine.

After analyzing health data from more than 200,000 Americans, researchers calculated that current smokers were three times more likely to die during the course of the study compared with people who had never smoked.

For the most part, their deaths were caused by smoking-related ailments, including heart and lung disease. Overall, their odds of surviving to age 80 were half as good as for never-smokers.

But the study, one of two large-scale surveys in the journal providing updated information on smoking and mortality, saw significant benefits for those who quit. Giving up smoking between the ages of 35 and 44 was associated with a gain of nine years of life, and those who quit between 45 and 54 lived an extra six years.

"The good news is, because the risks are so big, the benefits of quitting are quite substantial," said study leader Prabhat Jha, an epidemiologist and director of the Center for Global Health Research, based in Toronto.

While the U.S. smoking rate has declined to 19.3 percent among adults, there are still an estimated 45.3 million smokers in the country, according to the Centers for Disease Control and Prevention. Cigarette use is responsible for about 443,000 U.S. deaths each year, the CDC says.

Using the National Health Interview Survey, the researchers followed 113,752 women and 88,496 men in the United States between 1997 and 2004, categorizing them as smokers (at least 100 cigarettes within their lifetime), former smokers (no smoking within the last five years) and never-smokers. Former smokers were held to the five-year rule in order to weed out those who were already in declining health because of potentially fatal smoking-related diseases.

The researchers checked death records in 2006 and found that 8,236 of the women and 7,479 of the men had died. By comparing mortality rates among the groups, Jha's team calculated that women between the ages of 25 and 79 who were current smokers were three times more likely to die than women who never smoked.

Among men in that age group, those who still smoked were 2.8 times more likely to die than never-smokers. The results were adjusted for age, education, body-mass index and alcohol consumption, since smokers tended to be thinner, have less education and be more likely to drink.

The second study examined mortality rates over half a century in 2.2 million people 55 and older -- possibly the largest such survey undertaken, said lead author Michael Thun, a recently retired cancer epidemiologist with the American Cancer Society.

Thun's survey measured trends in death rates across three time periods: 1959 to 1965, 1982 to 1988 and 2000 to 2010.

The analysis revealed a worrying trend that also cropped up in Jha's study: Women's death rates from smoking, which had long lagged behind men, had pulled even.

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